Field of the Invention
The invention relates to a magnetic-inductive flowmeter for measuring the flow of a flowing medium, having a measuring tube, having a magnetic field generator for generating a magnetic field at least partially interfusing the measuring tube and at least one measuring electrode for tapping a measuring voltage induced in the flowing medium. Furthermore, the invention relates to a method for producing a measuring electrode, which is suitable, in particular for a magnetic-inductive flowmeter.
Description of Related Art
According to Faraday's law of induction, an electric field strength is formed perpendicular to the direction of flow of the medium and perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field. Faraday's law of induction is thus exploited in magnetic-inductive flowmeters in that a magnetic field fluctuating over time during the measurement process is usually generated by means of a magnetic field generator usually having at least one magnetic field coil, and that the magnetic field at least partially interfuses the electrically conductive medium flowing through the measuring tube. Here, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
Magnetic-inductive flowmeters have at least one measuring electrode for tapping a measuring voltage induced in an electrically conductive medium. In some implementations of the prior art, at least two measuring electrodes are present. These measuring electrodes preferably come into contact with the medium and the virtual connection line of the two measuring electrodes runs at least essentially perpendicular to the direction of the magnetic field interfusing the measuring tube perpendicular to the longitudinal axis of the measuring tube.
Magnetic-inductive flowmeters known from the prior art in German Patent DE 692 32 633 C2, DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754 B1, German Patent Application DE 100 64 738 B4 and corresponding U.S. Pat. No. 6,564,612 B2, German Patent Application DE 102 43 748 A1 and corresponding U.S. Pat. No. 6,804,613 B2, German Patent Application DE 10 2008 005 258 A1 and corresponding U.S. Pat. No. 7,971,493 B2 and German Patent Application DE 10 2011 112 703 A1 and corresponding U.S. Patent Application Publication 2012/0066301 A1 as well as EP 0 704 682 A1 and EP 0 834 057 A1 and corresponding U.S. Pat. No. 6,092,428 are referred to as examples. In particular, reference is made to the documents that concern problems that are relevant for magnetic-inductive flowmeters in conjunction with the measuring tubes or with the measuring electrodes, namely to German Patent Application DE 102 40 024 A1 and corresponding U.S. Pat. No. 6,843,136 B2, German Patent Application DE 10 2008 059 067 A1 and corresponding U.S. Pat. No. 8,074,525 B2, German Patent Application DE 10 2010 056 077 A1, DE 10 2012 017 904 A1 and corresponding U.S. Pat. No. 9,121,740 B2 and German Patent Application DE 10 2014 001 479 A1.
German Patent Application DE 102 40 024 A1 and corresponding U.S. Pat. No. 6,843,136 B2 describes a magnetic-inductive flowmeter, in which the measuring tube has measuring electrode channels led through its walls and the measuring electrodes are arranged in the measuring electrode channels in such a manner that their measuring electrode head is set back from the inner wall of the measuring tube. It is provided thereby that the respective inner space of the measuring electrode channel in front of the measuring electrode head is free up to the inner space of the measuring tube. In this manner, an improved signal-to-noise ratio of the measuring voltage tapped at the measuring electrodes is achieved.
In the magnetic-inductive flowmeter known from German Patent Application DE 10 2008 059 067 A1 and corresponding U.S. Pat. No. 8,074,525 B2, the measuring tube is provided at least partially with an electrically insulating protective layer on the inside. Thereby, the measuring electrodes are designed in a stripped manner and have a length—in the circumferential direction of the measuring tube—of slightly less than a quarter of the measuring tube circumference to slightly more than half of the measuring tube circumference.
German Patent Application DE 10 2010 056 077 A1 describes a magnetic-inductive flowmeter, in which the measuring tube has an insulated lining on its inner wall and the measuring electrodes consist at least in part of an electrically conductive material and have a bonded connection on the inner wall with the lining.
A magnetic-inductive flowmeter is known from German Patent Application DE 10 2012 017 904 A1 and corresponding U.S. Pat. No. 9,121,740 B2, in which the measuring tube has a metallic body and the body is provided with a thermoplastic protective layer at least on the inside of the measuring tube. Thereby, a liquid-tight connection between the thermoplastic protection layer of the measuring tube and the measuring electrodes caused by heating the protection layer is implemented at the penetration points, at which the measuring electrodes penetrate the measuring tube. Finally, a magnetic-inductive flowmeter is known from German Patent Application DE 10 2014 001 479 A1, in which the measuring tube has a circular penetration point, a support surface following the penetration point provided on the measuring tube and at least one fixing element provided spaced relative to the support surface for each measuring electrode and each measuring electrode has an electrode shaft extending through or jutting into the penetration point and an electrode head extending over the electrode shaft.